1. Field of the Invention
The present invention generally relates to emulsion ink and printing methods using such ink, and particularly, to emulsion ink having an excellent fixing characteristic for a printing matter and which does not bleed or strike through and which is suitable, especially, for a stencil printing.
2. Description of the Related Art
A stencil printing method, as widely known, is a printing method in which an image pattern made of a number of fine pores is formed on stencil paper. Printing onto a print material such as paper is performed using the stencil paper as a plate and moving ink through the fine pores from one side of the plate to the other side.
There are two types of rotary stencil duplicator--one is a double drum type having a blanket and the other is a single drum type having no blanket. As for the former, soft ink as well as hard ink may be used since there is no case where more than necessary amount of ink is supplied onto print paper. As for the latter, on the other hand, more than a necessary amount of ink may easily be supplied onto print paper if soft ink is used. The ink may easily strike through the paper or leak to an end portion of a master stencil and may cause a mess on a press roller or printed material.
Also, conventional emulsion ink contains solvents having a high boiling point, such as nondrying oil and non-volatile oil (motor oil, liquid paraffin, spindle oil, machine oil, caster oil, etc.) in order to avoid clogging of the pores which could happen if the ink is dried on the stencil plate. However, if the ink is left in a printing machine for a certain period, only water contained in the ink is evaporated and the balance of aqueous phase and organic phase in the ink is disturbed. As a result, the viscosity of the ink is lowered and the ink is softened. Thus, a printing concentration becomes higher than normal and problems such as the bleeding or striking through of the ink are caused.
In order to modify the above mentioned characteristic of the conventional ink, many proposals has been offered and practically carried out. For example, as disclosed in the Japanese Laid-Open Patent Application No.63-255967, the balance of the aqueous and organic phases of ink may be maintained by containing a solvent having an intermediate boiling point in the ink so that the organic phase is also evaporated as the water contained in the ink is evaporated. In the above Japanese Laid-Open Patent Application, oxy-carboxylic acid is used for the purpose so that the viscosity of the ink may be properly maintained. In the Japanese Laid-Open Patent Application No.63-277287, the above mentioned problems are avoided by properly setting a thixo coefficient and the viscosity of ink. In the Japanese Laid-Open Patent Application No.1-318073, the problems are prevented by adding acryl resin powder to an oil-based ultraviolet cure type ink having no water content. Similarly, in the Japanese Laid-Open Patent Application No.2-41376, silicone oil is added to the oil-based ultraviolet cure type ink having no water content. Moreover, improvement is made using foam type ink in the Japanese Laid-Open Patent Application No.2-86667.
In the Japanese Laid-Open Patent Application No.4-233980, an improvement in the stability of ink is made by gelatinizing an organic phase to give it a thixotropic characteristic so that the viscosity of emulsion ink is not significantly changed even if the water in the ink is evaporated. In the Japanese Laid-Open Patent Application No.5-70729, a salt of pyrrolidone carboxylic acid derivative having a high hygroscopic characteristic is added to ink to control the water evaporation and an oil whose surface tension is low is used as one of the components for the organic phase in order to improve permeability of the ink to printing paper by increasing the wettability of the ink without changing the viscosity. In the Japanese Laid-Open Patent Application No.5-125320, a volatile solvent whose first distillation point is between about 150 to 210.degree. C. is contained in the organic phase. In the Japanese Laid-Open Patent Application No.5-247397, ink having a low yield value and low flowability at high shear is produced by adding cyclized rubber to the ink. In the Japanese Laid-Open Patent Application No.6-128516, a heat melting component, a thermosetting component and silicone oil are added to ink to improve drying and fixing characteristics of the ink. In the Japanese Laid-Open Patent Application No.6-172691, a wax component is added to ink to avoid the above mentioned problems.
However, despite these improvements, the above mentioned problems regarding conventional ink have not been completely solved. Moreover, printing machines and peripheral equipments have greatly progressed these days with the appearance of high technology such as computers and demands for solving the problems mentioned above are rapidly increasing.